How to get illumination pass from a Smoke Sim with Cycles?
One solution is to add a material index to your fire's material, and then, on the compositor,  you can take the Emission pass, and multiply it with the Object Index one.



Go to the Properties panel, then switch to the Materials tab, and under the Settings subsection, change the material index.



Then, still on the Properties panel, switch to the Render Layers tab, go to the Passes subsection and activate Material Index and Emission.



Then, in the compositor, multiply the output of a IDMask set to 1 and the Emission pass.